(1) Field of the Invention
The present invention relates to an injection-molded body of an olefin/vinyl alcohol copolymer having such a high molecular weight as not attainable by the conventional techniques and hence, being excellent in various characteristics, and a process for the preparation thereof.
(2) Description of the Prior Art
It is known that olefin/vinyl alcohol copolymers represented by ethylene/vinyl alcohol copolymers are thermoplastic polymers which can be melt-extruded and are excellent in the gas barrier properties, as disclosed in the specification of U.S. Pat. No. 3,419,645.
It also is known that by utilizing these characteristics, an olefin/vinyl alcohol copolymer is used in the form of a single layer or a laminate with other resin layer for imparting gas barrier properties to a packaging material such as a packaging vessel or film, and many proposals have been made in this field.
The extrusion molding method and injection molding method are generally adopted for forming olefin/vinyl alcohol copolymers into packaging materials, and in the production of a packaging material having a multi-layer structure, there are adopted a method in which extruders are used in the number corresponding to the number of kinds of resins used and the resins are co-extruded through a multi-layer multi-ply die, and a method in which injection machines are used in the number corresponding to the number of kinds of resins used and the resins are injected into an injection mold sequentially or simultaneously.
For an ethylene/vinyl alcohol copolymer, there is a suitable range of the molecular weight according to the molding method. For example, in case of injection molding, in view of the melt flowability and the precision of a molded article, the weight average molecular weight (Mw) is relatively low and in the range of from 11,000 to 59,000 (corresponding to the melt flow rate of 3.0 to 60 g/10 min), and in case of extrusion molding for obtaining a blow-formed vessel, in order to attain a good shape-retaining property and prevent thickness unevenness in a parison, the weight average molecular weight is relatively high and generally in the range of 53,000 to 78,000 (corresponding to the melt flow rate of 0.4 to 6.0 g/10 min).
Japanese Patent Application Laid-Open No. 189407/85 or No. 193614/85 discloses a technique of preparing a multi-layer structure comprising a layer of an ethylene/vinyl alcohol copolymer as the intermediate layer by injection molding, but the ethylene/vinyl alcohol copolymer used in this technique has a low molecular weight.
It seems strange that an ethylene/vinyl alcohol copolymer having a high molecular weight, for example, a weight average molecular weight (Mw) higher than 80,000, especially higher than 90,000, has not been used. It is considered that the reason is that an ethylene/vinyl alcohol copolymer having such a high molecular weight involves difficulties in the polymerization operation, the saponification operation and the yields at these steps and molding of the ethylene/vinyl alcohol copolymer having such a high molecular weight is difficult.
In general, the physical properties of a thermoplastic polymer are greatly influenced by the molecular weight, and it is known that such properties as tensile strength, impact strength, creep resistance and rigidity are improved as the molecular weight is increased. Also in an ethylene/vinyl alcohol copolymer, it is expected that increase of the molecular weight will result in improvement of the foregoing characteristics.